Well he left so whatever is coming next, AI related or not, "small" or not (small for them might be reaching just a million people, he wrote that he "lead the software layer that powers the entire AI industry." so his notion of scale is probably unlike mine, maybe yours too) is more exciting to him that whatever he could do next with all of Meta's resources.

Edit: to be clear, I didn't mean to imply their next thing is AI related, solely that they obviously know more about AI at Meta than e.g. XR at Meta, just because that's their expertise.